Precision robotics

Most users of robot manipulators in an industrial setting are content with the excellent repeatability of their movements. Today, an industrial robot’s effector can reach a previously taught pose (position and orientation) with a position error below 0.1 mm. However, in some applications, the poses to be attained are computed rather than taught, in which case absolute accuracy is also required.

The most cost-effective way of improving the absolute accuracy of robots is through calibration. While various commercial solutions are indeed available for calibrating robots using metrology devices, the fact is that even the least expensive of them still cost several thousand dollars. Furthermore, for various reasons, these solutions do not meet the needs of all users (lack of space in a robotic cell, lack of budget to buy a three-dimensional measurement device, etc.).

While the aerospace sector is probably the most demanding in terms of precision robotics, it is paradoxically also the least well served. This sector has a particularly pronounced and active presence in the Montreal region, where several international companies, including Pratt & Whitney, GE Aviation, Messier-Dowty, L-3 MAS and Bombardier Aerospace, use industrial robots for precision tasks.
 
Development of robot calibration methods

The CoRo laboratory is equipped with a wide range of metrology devices:
  • three-dimensional measuring machine (Mitutoyo)
  • laser interferometer system (Renishaw)
  • laser tracker (Faro)
  • measuring arm (Faro)
  • ballbar (Renishaw)
  • probe (Renishaw)
     
The laboratory is also equipped with two serial as well as several parallel robots. Robot calibration projects consist essentially in using measurement devices and mechanical artefacts to optimize the absolute accuracy of a robot.  This type of research work requires a very strong mathematical background, and knowledge of robot kinematics and optimization methods.

The first method developed at the CoRo laboratory is based on the use of a laser tracker to calibrate a classical six-degrees-of-freedom serial robot. Experimental work was carried out on an ABB IRB 1600 robot, and its maximum positioning error in its entire workspace was reduced to below 0.850 mm. Measurements were conducted automatically by controlling the laser tracker and the robot through a local Ethernet network, from Matlab.


Calibration of a serial robot with a laser tracker (vidéo)

A second method has been developed to calibrate one of the parallel robots designed at the laboratory, PreXYT, using a measuring arm (or any 3D coordinate measuring machine). This robot’s maximum positioning error has been reduced to below 0.050 mm.


Calibration of a parallel robot with a measurement arm (video)

Other calibration approaches have also been explored, including the calibration of serial robots using a probe, and the calibration of a Moog hexapod using a laser tracker.

The CoRo laboratory is currently exploring the use of C-Track, a dual-camera measurement device by the Quebec manufacturer, Creaform. This optical device allows the real-time measurement of the robot's effector pose, and can thus possibly be used not only to calibrate robots, but also to guide them dynamically.
 

Design of parallel robots for precision positioning

R&D activities pertaining to the design of parallel robots for precision positioning are described in the Parallel robotics section.